There is a general need for heating subsea pipelines.
Oil/gas installations may typically have multiple wells tied back to a single floating structure, located near the wells. Unprocessed hydrocarbon (crude oil/gas) exits the well at high temperature and high pressure, and must be kept warm to prevent hydrates or wax from clogging up the pipeline. Pipelines are therefore thermally insulated to ensure flow during normal production. There can be many kilometers between the well and the platform, and cold, surrounding seawater leads to heat loss in the flowing hydrocarbon. In the event of production shut-down, the hydrocarbon in the pipeline will cool over time. This can lead to the formation of wax or hydrate plugs when the temperature drops below a critical level. Such formation of wax or hydrate should be avoided.
Direct Electrical Heating (DEH) systems have been developed to eliminate or alleviate problems associated with wax or hydrate plug formation. In a typical DEH system, a feeder cable supplies power from the topside (e.g., a platform) down to the sea bed. The feeder cable is connected to the pipeline and to a piggyback cable which extends along the pipeline. The feeder cable typically contains two conductors, one connected to the pipeline at the platform end and the other connected to the piggyback cable. Electric current is conducted through the piggyback cable and the conductive material (typically steel) of the pipeline, in parallel with surrounding sea water. Heat is directly generated in the pipeline material due to the material's resistive properties.
Alternating current (AC) is used to supply DEH systems. Since a typical DEH system constitutes a highly inductive load (its power factor is low), a capacitor arrangement, usually arranged topside, has previously been provided to compensate for the inductive load, i.e. to reduce the amount of reactive power. If a capacitor is located subsea this allows for reduced dimensions of conductors in the feeder cable. However, such a capacitor arrangement involves increased costs, is usually bulky and heavy, and represents a possible source of failure.
WO-2013/124270 describes an example of a DEH system, wherein a capacitor arrangement is provided to balance the inductive load with the power supply. The capacitor arrangement may be located topside or subsea.